Browsing by Subject "FTIR"

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Open Access
Analysis of UV induced dehydrochlorinated PVC (With hydroquinone) using direct pyrolysis mass spectrometry (DPMS), TGA, UV/VIS-NIR and FTIR techniques
(2003) Avcı, Ercan
Poly(vinyl chloride) (PVC) degrades easily upon heat and light exposure via loss of HCl. The mechanism of this process is well understood, known as the zip mechanism and the dehydrochlorination results in conjugated segments, polyenes. It is also possible to utilize PVC polymer as an in-situ acid donor since the main degradation product is HCl. Addition of hydroquinone (HQ) into PVC matrix sensitizes the photodehydrochlorination of PVC at 312 nm. In this study the effects of photodehydrochlorination on thermal and material properties of PVC were investigated using DPMS and TGA as well as UV-Vis-NIR and FTIR techniques. In addition, the photodegradation of PVC/PVAc blend, copolymer (PVC-coPVAc) and PVAc were similarly investigated. Dehydrochlorination of the polymers resulting from UV-exposure were also investigated for doping of PANI in blends. HCl evolution behavior of the UV dehydrochlorinated PVC exhibits a characteristic property which is different from the unirradiated ones. Both DPMS and TGA results confirms the sensitization of PVC photodehydrochlorination at 312 nm by hydroquinone (HQ) resulting in a temperature onset that is the lowest (140 oC). HQ assistance upon 312 nm UV exposure is not significantly observed in the copolymer. The low temperature onset of UV-induced copolymer is a promising result to produce longer polyene chains, since polymer backbone starts to decompose after ca. 220 oC, using copolymer might be an alternative to PVC.
Open Access
Effect of reactor pressure on optical and electrical properties of InN films grown by high-pressure chemical vapor deposition
(Wiley - V C H Verlag GmbH & Co. KGaA, 2015) Alevli, M.; Gungor, N.; Alkis, S.; Ozgit Akgun, C.; Donmez, I.; Okyay, Ali Kemal; Gamage, S.; Senevirathna, I.; Dietz, N.; Bıyıklı, Necmi
The influences of reactor pressure on the stoichiometry, free carrier concentration, IR and Hall determined mobility, effective optical band edge, and optical phonon modes of HPCVD grown InN films have been analysed and are reported. The In 3d, and N 1s XPS spectra results revealed In-N and N-In bonding states as well as small concentrations of In-O and N-O bonds, respectively in all samples. InN layers grown at 1 bar were found to contain metallic indium, suggesting that the incorporation of nitrogen into the InN crystal structure was not efficient. The free carrier concentrations, as determined by Hall measurements, were found to decrease with increasing reactor pressure from 1.61×1021 to 8.87×1019 cm-3 and the room-temperature Hall mobility increased with reactor pressure from 21.01 to 155.18 cm2/Vs at 1 and 15 bar reactor pressures, respectively. IR reflectance spectra of all three (1, 8, and 15 bar) InN samples were modelled assuming two distinct layers of InN, having different free carrier concentration, IR mobility, and effective dielectric function values, related to a nucleation/interfacial region at the InN/sapphire, followed by a bulk InN layer. The effective optical band gap has been found to decrease from 1.19 to 0.95 eV with increasing reactor pressure. Improvement of the local structural quality with increasing reactor pressure has been further confirmed by Raman spectroscopy measurements. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
Open Access
Fe promoted NOx storage materials: structural properties and NOx uptake
(American Chemical Society, 2010) Kayhan, E.; Andonova, S. M.; Şentürk, G. S.; Chusuei, C. C.; Ozensoy, E.
Fe promoted NOx storage materials were synthesized in the form of FeOx/BaO/Al2O3 ternary oxides with varying BaO (8 and 20 wt %) and Fe (5 and 10 wt %) contents. Synthesized NOx storage materials were investigated via TEM, EELS, BET, FTIR, TPD, XRD, XPS, and Raman spectroscopy, and the results were compared with the conventional BaO/Al2O3 NOx storage system. Our results suggest that the introduction of Fe in the BaO/Al2O3 system leads to the formation of additional NOx storage sites which store NOx mostly in the form of bidentate nitrates. NO2 adsorption experiments at 323 K via FTIR indicate that, particularly in the early stages of the NOx uptake, the NOx storage mechanism is significantly altered in the presence of Fe sites where a set of new surface nitrosyl and nitrite groups were detected on the Fe sites and the surface oxidation of nitrites to nitrates is significantly hindered with respect to the BaO/Al2O3 system. Evidence for the existence of both Fe3+ as well as reduced Fe2+/(3-x)+ sites on the freshly pretreated materials was detected via EELS, FTIR, Raman, and XRD experiments. The influence of the Fe sites on the structural properties of the synthesized materials was also studied by performing ex situ annealing protocols within 323-1273 K followed by XRD and Raman experiments where the temperature dependent changes in the morphology and the composition of the surface domains were analyzed in detail. On the basis of the TPD data, it was found that the relative stability of the stored NOx species is influenced by the morphology of the Ba and Fe containing NOx-storage domains. The relative stabilities of the investigated NOx species were found to increase in the following order: N2O3/NO+ < nitrates on γ-Al2O3 < surface nitrates on BaO < bidentate nitrates on FeOx sites < bulk nitrates on BaO.
Open Access
Finding an optimum surface chemistry for [Formula] systems as NOx storage materials
(2010) Şentürk, Göksu Seda
Titania promoted NOx storage materials in the form of BaO/TiO2/Al2O3 were synthesized via two different sol-gel preparation techniques, with varying surface compositions and morphologies [1, 2]. The influence of the TiO2 units on the NOx storage component (8 - 20 wt. % BaO), the nature of the crystallographic phases, thermal stabilities and the dispersion of the surface oxide/nitrate domains were investigated. The structural characterization of the synthesized NOx storage materials were analyzed by means of BET surface area analysis, X-ray diffraction (XRD), ex-situ Raman spectroscopy, scanning electron microscopy (SEM), energy dispersive X- ray (EDX) and transmission electron microscopy (TEM). Comparative analysis of the results showed that the TiO2/Al2O3 support material derived by the co-precipitation of the corresponding hydroxides via the sol-gel technique, exhibited distinctively more homogenous distribution of TiO2 domains. The functionality/performance of these materials upon NOx and SOx adsorptions were monitored by temperature programmed desorption (TPD) and insitu Fourier transform infrared (FTIR) spectroscopy. An improved Ba surface dispersion was observed for the BaO/TiO2/Al2O3 materials synthesized via the coprecipitation of alkoxide precursors which was found to originate mostly from the increased fraction of accessible TiO2/TiOx sites on the surface. These TiO2/TiOx sites functioned as strong anchoring sites for surface BaO domains and were tailored to enhance surface dispersion of BaO. The relative stability of the NOx species adsorbed on the BaO/TiO2/Al2O3 system was found to increase in the following order: NO+ /N2O3 on alumina << nitrates on alumina < surface nitrates on BaO < bridged/bidentate nitrates on large/isolated TiO2 clusters < bulk nitrates on BaO on alumina surface and bridged/bidentate nitrates on TiO2 crystallites homogenously distributed on the surface < bulk nitrates on the BaO sites located on the TiO2 domains. The detailed study of the interaction of SOx with BaO/TiO2/Al2O3 ternary oxide materials showed that titania (TiO2) was a promising candidate for improving the sulfur tolerance on these type of surfaces. Adsorption of SOx on both pure Al2O3 and TiO2 showed that Al2O3 formed strongly bound SOx species, that were thermally stable up to T > 1073 K. SOx adsorption directly altered stability of the nitrate species on the Ti/Al (Protocol 1, Protocol 2) samples. SOx uptake properties of the BaO/TiO2/Al2O3 materials were found to be strongly influenced by the morphology of the TiO2/TiOx domains and the BaO loadings (8/20 wt% BaO). Consequently, the presence of titania domains was seen to decrease the SOx desorption temperatures and enhance the sulfur-tolerance of these materials by destabilizing the accumulated sulfate species. SOx exposure on the synthesized materials led to a significant decrease in the NOx adsorption capacities. The results obtained from FT-IR spectra showed that the sulfur deposition on the NOx storage materials promoted by Ti
Open Access
Germanium alloys for optoelectronic devices
(2008) Erbil, Ayşe
Silicon has been the backbone of the mainstream electronics of the last fifty years. It is however, used in conjunction with other matierals, mainly with its oxides and nitrides. Germanium, on the other hand, is also a group IV element and has been used in the early stages of transistor and detector development. In addition to Si/Ge heterojunctions, bandgap engineering through SiGe alloys has also been used in photodetectors. Recent progress in light emitting devices utilizing Si nanocrystals suggest the use of Ge1-xNx layers as barriers due to its suitable band offsets [1]. Experiments have shown that Ge1-xNx is also a promising material for applications in photodiodes, amplifiers, optic fibers, protective coatings, etc [1]. Both Si and Ge are, however indirect bandgap semiconductors, lacking efficient light emission. On the other hand, strong light emission observed in Si nanocrystals has made the study of semiconductor nanocrystals an expanding field of interest due to potential applications in novel optoelectronic devices [2]. These nanocrystals exhibit strong luminescence and nonlinear optical properties that usually do not appear in the bulk materials [2]. SiGe nanocrystals attract attention due to the possibility of a tunable band gap with composition. In this study, formation of Ge1-xNx thin films and SiGe nanocrystals by plasma enhanced chemical vapor deposition (PECVD) reactor has been studied. We present the growth conditions and experimental characterization of the resulting thin films and nanocrystals. We used ellipsometry, Raman Spectrometry, Fourier Infrared Spectrometry (FTIR) and X-ray photoelectron Spectroscopy (XPS). For SiGe nanocrystals, 4 peaks in the Raman Spectra were observed around 295 cm-1, 400 cm -1, 485 cm-1 and 521 cm-1. These peaks are assigned to the Ge-Ge, Si-Ge, local Si-Si and crystalline Si-Si vibrational modes, respectively [3]. For the Ge1-xNx thin films FTIR spectrum showed the existence of the Ge-N bonds and its band offsets determined by XPS confirm its suitability for optoelectronic devices.
Open Access
In-Situ vibrational spectroscopic studies on model catalyst surfaces at elevated pressures
(Springer Verlag, 2013) Ozensoy, E.; Vovk, E. I.
Elucidation of complex heterogeneous catalytic mechanisms at the molecular level is a challenging task due to the complex electronic structure and the topology of catalyst surfaces. Heterogeneous catalyst surfaces are often quite dynamic and readily undergo significant alterations under working conditions. Thus, monitoring the surface chemistry of heterogeneous catalysts under industrially relevant conditions such as elevated temperatures and pressures requires dedicated in situ spectroscopy methods. Due to their photons-in, photons-out nature, vibrational spectroscopic techniques offer a very powerful and a versatile experimental tool box, allowing real-time investigation of working catalyst surfaces at elevated pressures. Infrared reflection absorption spectroscopy (IRAS or IRRAS), polarization modulation-IRAS and sum frequency generation techniques reveal valuable surface chemical information at the molecular level, particularly when they are applied to atomically well-defined planar model catalyst surfaces such as single crystals or ultrathin films. In this review article, recent state of the art applications of in situ surface vibrational spectroscopy will be presented with a particular focus on elevated pressure adsorption of probe molecules (e.g. CO, NO, O-2, H-2, CH3OH) on monometallic and bimetallic transition metal surfaces (e.g. Pt, Pd, Rh, Ru, Au, Co, PdZn, AuPd, CuPt, etc.). Furthermore, case studies involving elevated pressure carbon monoxide oxidation, CO hydrogenation, Fischer-Tropsch, methanol decomposition/partial oxidation and methanol steam reforming reactions on single crystal platinum group metal surfaces will be provided. These examples will be exploited in order to demonstrate the capabilities, opportunities and the existing challenges associated with the in situ vibrational spectroscopic analysis of heterogeneous catalytic reactions on model catalyst surfaces at elevated pressures.
Open Access
Investigation of NO2 and SO2 adsorption/desorption properties of advanced ternary and quaternary mixed oxides for DENOx catalysis
(2015-11) Say, Zafer
The main premise of the current study is the design, synthesis and functional characterization of novel catalytic materials with superior resistance against sulfur poisoning without compromising NOx storage capacity (NSC) in their NOx Storage Reduction (NSR) catalytic applications. BaO/TiO2-based materials are well known systems in deNOx catalysis, exhibiting promising performance towards sulfur poisoning. However, they suffer from limitations due to poor NSC and high affinity towards unwanted solid state interactionsbetweenTiO2 and BaO storage domains leading to the formation of BaTiOx.The main emphasis of the current work is the design of a novel catalytic system where ZrO2 and Al2O3 act as diffusion barriers between BaO and TiO2 domains while allowing good dispersion and preservation of the individual characteristicsof these active sites within a wide operational temperature window. Along these lines, binary and ternary mixed oxide materials, ZrO2/TiO2 (ZT) and Al2O3/ZrO2/TiO2 (AZT), and their Pt, BaO and/or K2O functionalized counterparts in the form of Pt/ZT, Pt/AZT, Pt/BaO/AZT, Pt/K2O/AZT and Pt/K2O-BaO/AZT with different mass loadings (i.e. 8 and 20 wt. % 20 BaO and 2.7, 5.4 and 10 wt. % K2O) were synthesized via sol-gel synthesis. Surface structure and catalytic properties of the synthesized materials were comprehensively investigated at the molecular level as a function of calcination temperature, catalyst composition, nature of the gas phase adsorbates (e.g. NO2, SO2, O2, H2, N2, N2O C5H5N etc.) interacting with the catalyst surface at various operational temperatures by means of XRD, Raman spectroscopy, BET analysis, in-situ FTIR and TPD. Current results indicate no evidence for the formation of undesired BaTiOx and/or KTiOx. NSC of fresh monolithic catalysts was also quantitatively measured under realistic operational conditions in a tubular flow reactor system. These flow reactor measurements indicated that Pt/8BaO/AZT and Pt/20BaO/AZT materials revealed promising NOx storage and sulfur regeneration performance at low (i.e. 473 K) and moderate (i.e. 573 K) temperatures in comparison to the conventional Pt/20Ba/Al2O3 benchmark catalyst. However, they were found to be surpassed by the conventional Pt/20BaO/Al2O3 benchmark catalyst at higher operational temperatures (i.e. 673 K). Therefore, activity loss at high temperatures was alleviated by incorporating a high-temperature storage functionality (i.e. K2O) to the catalyst structure. Upon this structural enhancement, Pt/5.4K2O/AZT catalyst was found to reveal much higher NSC at high temperatures (i.e. 673 K) as compared to BaO-based materials. An overall assessment of the results presented in the current study suggests that there exists a delicate trade-off between NOx Storage Capacity (NSC) and sulfur uptake/poisoning in NSR systems which is strongly governed by the BaO and K2O loading/dispersion as well as the surface structure of the support material.
Open Access
NOx reduction on a transition metal-free γ-Al2O3 catalyst using dimethylether (DME)
(2008) Ozensoy, E.; Herling, D.; Szanyi, J.
NO2 and dimethylether (DME) adsorption as well as DME and NO2 co-adsorption on a transition metal-free γ-alumina catalyst were investigated via in-situ transmission Fourier transform infrared spectroscopy (in-situ FTIR), residual gas analysis (RGA) and temperature programmed desorption (TPD) techniques. NO2 adsorption at room temperature leads to the formation of surface nitrates and nitrites. DME adsorption on the alumina surface at 300 K leads to molecularly adsorbed DME, molecularly adsorbed methanol and surface methoxides. Upon heating the DME-exposed alumina to 500-600 K the surface is dominated by methoxide groups. At higher temperatures methoxide groups are converted into formates. At T > 510 K, formate decomposition takes place to form H2O(g) and CO(g). DME and NO2 co-adsorption at 423 K does not indicate a significant reaction between DME and NO2. However, in similar experiments at 573 K, fast reaction occurs and the methoxides present at 573 K before the NO2 adsorption are converted into formates, simultaneously with the formation of isocyanates. Under these conditions, NCO can further be hydrolyzed into isocyanic acid or ammonia with the help of water which is generated during the formate formation, decomposition and/or NCO formation steps. © 2008 Elsevier B.V. All rights reserved.
Open Access
Plasma enhanced chemical vapor deposition of low-loss as-grown germanosilicate layers for optical waveguides
(SPIE, 2004) Ay, Feridun; Agan, S.; Aydınlı, Atilla
We report on systematic growth and characterization of low-loss germanosilicate layers for use in optical waveguides. Plasma enhanced chemical vapor deposition (PECVD) technique was used to grow the films using silane, germane and nitrous oxide as precursor gases. Chemical composition was monitored by Fourier transform infrared (FTIR) spectroscopy. N-H bond concentration of the films decreased from 0.43 ×1022 cm -3 down to below 0.06x 1022 cm-3, by a factor of seven as the GeH4 flow rate increased from 0 to 70 seem. A simultaneous decrease of O-H related bonds was also observed by a factor of 10 in the same germane flow range. The measured TE loss rates at λ=632.8 nm were found to increase from are 0.20 ± 0.02 to 6.46 ± 0.04 dB/cm as the germane flow rate increased from 5 to 50 seem, respectively. In contrast, the propagation loss values for TE polarization at λ-1550 nm were found to decrease from 0.32 ± 0.03 down to 0.14 ± 0.06 dB/cm for the same samples leading to the lowest values reported so far in the literature, eliminating the need for high temperature annealing as is usually done for these materials to be used in waveguide devices.
Open Access
Role of organic and inorganic additives on the assembly of CTAB-P123 and the morphology of mesoporous silica particles
(2009) Poyraz, A. S.; Dag, Ö.
Mesoporous silica particles with various morphologies and structures have been synthesized by controlling the solubility, micellization, and assembly of a charged surfactant (cethyltrimethylammonium bromide, CTAB) and a pluronic (PEO20PPO70PEO20, P123) couple using an organic (benzene) or an inorganic (SO4 2-, NO3 -, or Cl-) additive. The effect of CTAB, with or without one of the Hofmeister ions or benzene in various concentrations, on the morphology, pore-size, pore-structure and the nature of the silica particles has been investigated. Increasing the lyotropic anion (SO4 2-) or benzene concentration of the synthesis media creates wormlike particles with enlarged pores and reduced wall thickness. However, the hydrotropic anion (NO3 -) influenced the solubility of the charged surfactant and increased the CTAB concentration in the CTAB-P123 micelles, and as a result, in the mesoporous silica particles. The surface area, unit cell, and pore size of the silica particles are diminished by increasing the nitrate ion centration. The effects of the Cl- ion are between the SO4 2- and NO3 -ions. It influenced the P123 at low and CTAB at high concentrations. At low CTAB/ P123 mol ratios, the Cl- ion affects mainly the P123, but at high CTAB/P123 it affects both the CTAB and P123. By carefully adjusting these ingredients (CTAB, SO4 2-, Cl-, NO3 - and benzene), not only the morphology of the particles, but also the pore-size and pore-structure of the mesoporous silica particles could be adjusted. The investigations were carried out by preparing a series of powder samples and, by varying the CTAB/P123 mol ratio (between 3.0 and 6.0) and the concentration of the organic (0.17 to 0.90 M) or inorganic (at 0.25, 0.50, or 1.00 M) additive in the synthesis media. The powder samples were analyzed using microscopy (SEM, TEM, and POM), diffraction (PXRD), and spectroscopy (FTIR, Raman, UV-vis, and EDS) techniques toward above goals. © 2009 American Chemical Society.
Open Access
Silicon based dielectrics : growth, characterization, and applications in integrated optics
(2005) Ay, Feridun
In recent years, growing attention has been paid to silicon based dielectrics, such as silicon oxynitrides, silicon nitrides, and semiconductor doped silicon oxides, all combined under the name silica on silicon technology. This attention has been motivated mainly due to their excellent optical properties such as well controlled refractive index and high transparency over a wide range of wavelength. In accordance with the main goal of this study that relied on the utilization of silicon based dielectrics and their optimization for applications in integrated optics, an emphasis was given to optimize the compositional and optical properties of these materials. A detailed quantitative compositional analysis using Fourier transform infrared spectroscopy resulted in identification of the germanosilicate dielectrics as the most promising candidates for use in integrated optics. The first reported systematic study of propagation losses for different-index planar waveguides by using prism coupling method was correlated with the compositional analysis. This study had an important outcome for planar waveguides fabricated with germanosilicate core layers resulting in the lowest propagation loss values reported so far for as deposited CVD-grown films at λ=1.55 µm, eliminating the need for costly and cumbersome annealing process. An improvement of the prism coupling technique led to a new approach for elasto-optic characterization of thin polymer films. This completely new method allows one to determine the optical anisotropy and out-of-plane mechanical properties and to correlate both in order to obtain the elasto-optical properties of thin polymer films, for the first time. Of interest as potential electro-optic material, we have concentrated on thermally poled germanosilicate films deposited on fused-silica substrates by PECVD. As a result of an optimization study, we demonstrated a record peak nonlinear coefficient of ∼1.6 pm/V, approximately twice as strong as the highest reliable value reported in a thermally poled fused silica glass. Finally, we have demonstrated several applications of this technology in the field of integrated optics. Since optical waveguides constitute the building blocks of many integrated optical devices, we had first concentrated on design and optimization of waveguides employing germanosilicates as the core layers. The final step of our work concentrated on design and implementation of microring resonator devices based on germanosilicate layers.
Open Access
Silicon oxynitride layers for applications in optical waveguides
(2000-09) Ay, Feridun
Silicon oxynitride layers, aimed to serve as the core material for optical waveguides operating at l.55µm, v.-ere grown by a PECVD technique using SiH4, N20, and NH3 as precursor gases. The films were deposited at 350 °c, 13.56 MHz RF frequency, and 1 Torr pressure by varying the flow rates of N20 and l\"H3 gases. The resulting refractirn indices of the layers varied between 1.47 and 2.0. The compositional properties of the layers were analyzed by FTIR and ATR infrared spectroscopy techniques. A special attention was given to the N-H bond stretching absorption at 3300-3400 cm-1, since its first overtone is known to be the main cause of the optical absorption at l.55µm. An annealing study was performed in order to reduce or eliminate this bonding type. For the annealed samples the corresponding concentration was strongly reduced as verified by FTIR transmittance and ATR methods. A correlation between the N-H concentration and absorption loss was verified for silicon oxynitride slab waveguides. Moreover, a single mode waveguide with silicon oxynitride core layer was fabricated. lts absorption and insertion loss values were determined by butt-coupling method, resulting in low loss waveguides.
Open Access
Spectroscopic characterization of VOx/ZrO2 catalysts prepared using vanadium(V) oxo complexes
(Elsevier, 2001) Kantcheva, M.
A method for deposition of vanadium(V) oxo species on zirconia using different vanadium(V) precursor ions is described. The samples are synthesized by suspension of the support (powder) in solutions containing: (i) the dioxovanadium(V) ion, VO2+; (ii) the yellow diperoxo anion, [VO(O-O)2]- and (iii) the red monoperoxo cation, [VO(O-O)]+. The application of vanadium(V) peroxo complexes significantly increases the uptake of vanadium by zirconia. The state and localization of the VOx species on the surface of zirconia have been studied by FTIR, UV-Vis and XP spectroscopies. © 2001 Elsevier Science B.V.
Open Access
Spectroscopic investigation of sulfur-resistant Pt/K2O/ZrO2/TiO2/Al2O3 NSR/LNT catalysts
(Elsevier BV, 2016) Say, Z.; Tohumeken, M.; Ozensoy, E.
An alternative ternary support oxide material and its K2O and Pt functionalized counterparts in the form of Pt/K2O/Al2O3/ZrO2/TiO2 with different K2O loadings were synthesized. Structural and morphological properties of the catalysts were characterized via XRD and BET techniques in comparison to a conventional Pt/20Ba/Al benchmark NSR/LNT catalyst. Comprehensive in-situ FTIR and TPD analysis revealed that increasing the K2O loading in the Pt/K2O/AZT system leads to an increase in NOx Storage Capacity (NSC) at the expense of the formation of bulk-like sulfates requiring higher temperature for complete sulfur elimination with H2(g). Observed delicate trade-off between NSC and sulfur poisoning tendencies of the currently investigated family of AZT-based NSR/LNT catalysts implies that Pt/5.4K2O/AZT is a promising catalyst revealing comparable NSC within the temperature range of 473-673 K to that of the conventional Pt/20Ba/Al benchmark catalyst, while exhibiting superior sulfur tolerance and regeneration characteristics.
Open Access
Structure control of silica-supported mono and bimetallic Au–Pt catalysts via mercapto capping synthesis
(Elsevier, 2013-02) Parola, V. L.; Kantcheva, M.; Milanova, M.; Venezia, A. M.
SiO2-supported monometallic and bimetallic platinum-gold catalysts are prepared by deposition of metal nanoparticles stabilized by mercaptopropyltriethoxysilane (MPTES) after different aging time of the solution containing metal ions and MPTES. The materials are tested in the hydrodesulfurization (HDS) reaction of thiophene and compared with corresponding catalysts prepared by the conventional deposition-precipitation (DP) method. The monometallic Pt and the bimetallic Au-Pt prepared by DP have comparable activity. With respect to the platinum catalyst prepared by DP, the corresponding platinum catalyst prepared by MPTES particle stabilization exhibits a substantial enhancement of the activity regardless the solution aging time. On the contrary, the MPTES-assisted Au-Pt catalysts have different activities, depending on the solution aging time, with the most active being the one obtained with the 5-day-aged solution. In accord with XRD, XPS, and FTIR, the aging time of the solution, through the different interaction of Pt or Au precursors with the mercapto groups, has a crucial effect on the structure and on the surface of the catalysts. The observed differences in the catalytic activity are related to the structural and compositional changes of the bimetallic particles.
Open Access
Sulfur poisoning and regeneration behavior of perovskite-based NO oxidation catalysts
(Springer New York LLC, 2017) Kurt M.; Say, Z.; Ercan, K. E.; Vovk, E. I.; Kim, C. H.; Ozensoy, E.
SOxuptake and release properties of LaMnO3, Pd/LaMnO3, LaCoO3and Pd/LaCoO3perovskites were investigated via in situ Fourier transform infrared (FTIR) spectroscopy, temperature programmed desorption and X-ray photoelectron spectroscopy. Sulfation of the perovskite leads to the formation of surface sulfite/sulfate and bulk-like sulfate species. Pd addition to LaMnO3and LaCoO3significantly increases the sulfur adsorption capacity. Pd/LaMnO3sample accumulates significantly more sulfur than LaMnO3; however it can also release a larger fraction of the accumulated SOxspecies in a reversible fashion at elevated temperatures in vacuum. This is not the case for Co-based materials, where thermal regeneration of bulk sulfates on poisoned LaCoO3and Pd/LaCoO3is extremely ineffective under similar conditions. However, in the presence of an external reducing agent such as H2(g), Pd/LaMnO3requires much lower temperature (873ï¿½K) for complete sulfur regeneration as compared to that of Pd/LaCoO3(973ï¿½K). Sequential CO and SOxadsorption experiments performed via in situ FTIR indicate that in the presence of carbonyls and/or carbonates, Pd adsorption sites may have a stronger affinity for SOxas compared to that of the perovskite surface, particularly in the early stages of sulfur poisoning.
Open Access
Theoretical and spectroscopic investigations on the structure and bonding in B-C-N thin films
(2009) Bengu, E.; Genisel, M. F.; Gulseren, O.; Ovali, R.
In this study, we have synthesized boron, carbon, and nitrogen containing films using RF sputter deposition. We investigated the effects of deposition parameters on the chemical environment of boron, carbon, and nitrogen atoms inside the films. Techniques used for this purpose were grazing incidence reflectance-Fourier-transform infrared spectroscopy (GIR-FTIR), X-ray photoelectron spectroscopy (XPS), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). GIR-FTIR experiments on the B-C-N films deposited indicated presence of multiple features in the 600 to 1700 cm- 1 range for the infrared (IR) spectra. Analysis of the IR spectra, XPS and the corresponding EELS data from the films has been done in a collective manner. The results from this study suggested even under nitrogen rich synthesis conditions carbon atoms in the B-C-N films prefer to be surrounded by other carbon atoms rather than boron and/or nitrogen. Furthermore, we have observed a similar behavior in the chemistry of B-C-N films deposited with increasing substrate bias conditions. In order to better understand these results, we have compared and evaluated the relative stability of various nearest-neighbor and structural configurations of carbon atoms in a single BN sheet using DFT calculations. These calculations also indicated that structures and configurations that increase the relative amount of C-C bonding with respect to B-C and/or C-N were energetically favorable than otherwise. As a conclusion, carbon tends to phase-segregate in to carbon clusters rather than displaying a homogeneous distribution for the films deposited in this study under the deposition conditions studied.
Open Access
Trade-off between NOx storage capacity and sulfur tolerance on Al2O3/ZrO2/TiO2–based DeNOx catalysts
(Elsevier, 2019) Say, Zafer; Mihai, O.; Kurt, Merve; Olsson, L.; Özensoy, Emrah
Al2O3/ZrO2/TiO2 (AZT) ternary mixed oxides functionalized with Pt and BaO were synthesized in powder and monolithic forms and were utilized in NOx Storage Reduction/Lean NOx Trap (NSR/LNT) catalysis as novel catalytic materials. Adsorption of NOx and SOx species and their interactions with the catalyst surfaces were systematically investigated via in-situ FTIR technique revealing different NOx coordination geometries governed by the presence and the loading of BaO in the powder catalyst formulation. While BaO-free Pt/AZT stored NOx as surface nitrates, BaO incorporation also led to the formation of bulk-like ionic nitrate species. NOx adsorption results obtained from the current Temperature Programmed Desorption (TPD) data indicated that NOx Storage Capacity (NSC) was enhanced due to BaO incorporation into the powder catalyst and NSC was found to increase in the following order: Pt/AZT < Pt/8BaO/AZT < Pt/20BaO/Al2O3 < Pt/20BaO/AZT. Increase in the NSC with increasing BaO loading was found to be at the expense of the formation of bulk-like sulfates after SOx exposures. These bulk-like sulfates were observed to require higher temperatures for complete regeneration with H2(g). Catalytic activity results at 473 K and 573 K obtained via flow reactor tests with monolithic catalysts suggested that Pt/AZT and Pt/8BaO/AZT catalysts with stronger surface acidity also revealed higher resistance against sulfur poisoning and superior SOx regeneration in spite of their relatively lower NSC. Monolithic Pt/ 20BaO/AZT catalyst revealed superior NSC with respect to the conventional Pt/20BaO/Al2O3 benchmark catalyst at 573 K after sulfur regeneration. On the other hand, this trend was reversed at high-temperatures (i.e. 673 K). Preliminary results were presented demonstrating the enhancement of the high-temperature NSC of AZTbased materials by exploiting multiple NOx-storage components where BaO functioned as the low/mid-temperature NOx-storage domain and K2O served as the high-temperature NOx storage domain. Enhancement in the high-temperature NOx-storage in the BaO-K2O multiple storage domain systems was attributed to the formation of additional thermally stable bulk-like nitrates upon K2O incorporation.